1. Field of Invention
The present general inventive concept relates to an apparatus and method to bombard a nucleus with charged particles so as to bring about a change in the nucleus resulting in a different isotope of the original nucleus or in a different element; and more particularly, to an apparatus to position a target material in the path of a stream of charged particles in order to produce a radioisotope for use in a radiopharmaceutical.
2. Description of the Related Art
A biomarker is used to interrogate a biological system and can be created by “tagging” or labeling certain molecules, including biomolecules, with a radioisotope. A biomarker that includes a positron-emitting radioisotope is required for positron-emission tomography (PET), a noninvasive diagnostic imaging procedure that is used to assess perfusion or metabolic, biochemical and functional activity in various organ systems of the human body. Because PET is a very sensitive biochemical imaging technology and the early precursors of disease are primarily biochemical in nature, PET can detect many diseases before anatomical changes take place and often before medical symptoms become apparent. PET is similar to other nuclear medicine technologies in which a radiopharmaceutical is injected into a patient to assess metabolic activity in one or more regions of the body. However, PET provides information not available from traditional imaging technologies, such as magnetic resonance imaging (MRI), computed tomography (CT) and ultrasonography, which image the patient's anatomy rather than physiological images. Physiological activity provides a much earlier detection measure for certain forms of disease, cancer in particular, than do anatomical changes over time.
A positron-emitting radioisotope undergoes radioactive decay, whereby its nucleus emits positrons. In human tissue, a positron inevitably travels less than a few millimeters before interacting with an electron, converting the total mass of the positron and the electron into two photons of energy. The photons are displaced at approximately 180 degrees from each other, and can be detected simultaneously as “coincident” photons on opposite sides of the human body. The modern PET scanner detects one or both photons, and computer reconstruction of acquired data permits a visual depiction of the distribution of the isotope, and therefore the tagged molecule, within the organ being imaged.
Most clinically-important positron-emitting radioisotopes are produced in a cyclotron. Cyclotrons operate by accelerating electrically-charged particles along outward, quasi-spherical orbits to a predetermined extraction energy generally on the order of millions of electron volts. The high-energy electrically-charged particles form a continuous beam that travels along a predetermined path and bombards a target. When the bombarding particles interact in the target, a nuclear reaction occurs at a sub-atomic level, resulting in the production of a radioisotope. The radioisotope is then combined chemically with other materials to synthesize a radiochemical or radiopharmaceutical suitable for introduction into a human body.
FIGS. 1 and 2 depict a conventional cyclotron used for the production of radioisotopes. As shown in FIG. 2, the cyclotron 6 includes an array of four “D” electrodes, also known as “dees” 61. The dees 61 are positioned in the valleys 62 of a large electromagnet 63. As shown in FIG. 1 and in the exploded view of the same cyclotron in FIG. 2, during operation of the cyclotron 6, an ion source continuously generates charged particles and introduces them into the cyclotron 6 at the center of the array of dees 61. The charged particles are exposed to a strong magnetic field generated by opposing magnet poles situated above and below the array of dees 61. A radio frequency (RF) oscillator applies a high frequency, high voltage signal to each of the dees 61 causing the charge of the electric potential developed across each of the dees 61 to alternate at a high frequency. Neighboring dees 61 are given opposite charges such that charged particles entering the gap between neighboring dees 61 see a like charge on one neighboring dee 61 and an opposite charge on the other neighboring dee 61, which results in acceleration (i.e., increasing the energy) of the charged particles. With each energy gain, the orbital radius of the charged particles increases. The result is a stream of charged particles A following an outwardly spiraling path away from the center of the array of dees 61. The charged particles ultimately exit the cyclotron 6 as a particle beam B directed at a target 11.
As shown in FIGS. 1 and 2, the particle beam B leaves the magnetic field of the cyclotron 6 before passing through a beam tube 91 and a collimator 93 to strike a target 11. The beam tube 91 and collimator 93 help to keep the particle beam focused after it leaves the magnetic field of the cyclotron 6. FIG. 3 shows an exploded view of a cyclotron 6′ in use with an internal target 11′—that is, a target that is positioned within the magnetic field of the electromagnet 63, so that a particle beam B′ generated by the cyclotron 6′ does not need to leave the magnetic field of the electromagnet 63 before striking the target 11′. Such an internal target has certain advantages over an external target. When using an external target, the particle beam loses some energy and concentrated power as it travels the distance between the cyclotron and the external target. Using an internal target, on the other hand, avoids this loss of beam energy and focus since the particle beam does not leave the immediate area of the cyclotron. This means that the particle stream generated by the cyclotron need not be as highly energetic as would be the case if it were necessary to compensate for a loss of beam energy and focus over distance. Therefore, using an internal target to position a target material in the path of the particle beam allows for the use of a smaller, less powerful cyclotron, with less attendant radiation and less need for shielding or extensive physical plant. Further, the elimination of the beam tube and collimator results in fewer total components contaminated by radiation.